72 CHEMICAL PROPERTIES OF SOIL 



rendered alkaline with ammonia and mixed with 

 ammonium oxalate solution. A white powdery pre- 

 cipitate indicates calcium, the presence of which is 

 confirmed by evaporating a little of each solution to 

 dryness, moistening with hydrochloric acid, and testing 

 on a platinum wire in the Bunsen burner. Calcium 

 compounds give a yellowish red flame colour. 



Limestone is also removed from the soil by the action of 

 carbonic acid, which produces soluble calcium bicarbonate : 



CaC03+ CO2+ H2O = Ca(HC03)2 



{U) About 50 grams of a calcareous soil are mixed with 

 water in a beaker, and carbon dioxide gas bubbled into 

 the mixture for ten minutes. The soil is filtered off, and 

 the filtrate boiled vigorously for a few minutes. A 

 white precipitate or turbidity indicates the presence of 

 the bicarbonate, which is decomposed by the action of 

 heat into normal calcium carbonate again : 



Ca(HC03)2 = CaC03+C02+H20. 



If no precipitate is obtained (indicating only a small 

 quantity of limestone in the soil), the filtrate should be 

 evaporated to dryness, taken up in a little dilute 

 hydrochloric acid, and tested for calcium as in {a) 

 above. 



CHAPTER VII 



Physical Properties of Soil 



Colour of Soil. 



The colour of any soil under examination should be noted, since 

 within certain limits it is an indication of fertility, black and red 

 soils being the most fertile. The colour is largely dependent on 

 the proportion of organic matter and iron compounds. 



